Motor Load Calculations, Part 4

In Part 3, we noted that you select the correct table from Tables 430.247, 430.248, 430.249, and 430.250 based on the title of the table. So let’s say you do that for a 480V, 100HP motor. Table 430.250 shows 124A as your Full Load Current. Is this the number you’ll use for feeder calculations?

Not necessarily. You must account for a couple of other things:

  1. Duty cycle. Multiply the nameplate current rating by the percentage shown in the table for the applicable duty cycle and motor duty rating.
  2. Multiple motors and/or other loads on the feeder. Follow the requirements of 430.24.

Let’s look more closely at that second item. If your motor is supplied by conductors that also supply other loads, each conductor must have an ampacity no smaller than the sum of each of the following:

  1. 125% of the full load current rating of the highest motor per 430.6(A).
  2. Sum of the full load current rating of the other motors per 430.6(A).
  3. 100% of the noncontinuous motor load.
  4. 125% of the noncontinuous motor load.

This seems straightforward enough. You’re basically adding up the loads while applying the 125% rule to the noncontinuous ones (as you would normally), plus multiplying the largest motor load by 125%.

But what if the largest motor is noncontinuous? Wouldn’t you be multiplying it twice by the 125% factor? The answer is no, and you’ll find that issue addressed in the three Exceptions listed below 430.24.

« Part 3 | Source: Mark Lamendola | Mindconnection